The present invention relates to combination weighing and is concerned in particular with an improvement in a combination weighing machine to increase the production rate without significantly sacrificing accuracy.
In a combination weighing machine, a plurality of weigh scales are loaded with a product in an amount that is generally a fraction of a target amount. The target amount may be either a total weight or a total number of articles obtained by dividing the scale weights by a unit weight factor. A search of the scales is made to select that combination which most closely approximates but is not less than the target amount. The scales which are members of the selected combination are then dumped to form a single charge of product which is delivered to an associated packaging machine or utilized in some similar manner.
Each cycle of operation in a combination weighing machine includes several sequential steps. The major steps are listed here together with typical elapsed time requirements for a 10-scale machine in the present state of the art.
______________________________________ (1) Deliver a new quantity of product to .200 secs. each empty scale (2) Allow newly loaded scales to settle .470 secs. (3) Read and process the weight of product .080 secs. in each scale (4) Conduct the combination search .050 secs. operation (5) Dump scales of best combination .200 secs. TOTAL 1.000 secs. ______________________________________
Packaging machines are typically required to form in each package a charge of product that is not less than a labeled weight. As a result, packaging machines are typically set to deliver charges of product that are slightly above the labeled weight, or target amount, in order to avoid any packaging violations. Therefore, each package may contain excess product that is traditionally called "give-away". For package weights of a few hundred grams, give-aways may be in the order of several grams, but state-of-the-art combination weighers have managed to reduce the give-away to one gram or less.
The give-away quantities in combination weighing machines stem from three major sources.
(1) Errors in the measured weights, PA1 (2) Statistical improbability of the best combination being exactly equal to the target amount, PA1 (3) Failure to deliver the full measured amount from a scale (scale adherence).
The present invention is concerned with item 2 above.
For efficient operation, a combination weighing machine must deliver charges of product with low give-away at high production speeds or rates. From a practical point of view, these two goals are in conflict. The component of give-away due to the statistical improbability mentioned in item 2 above, is reduced by a factor of 2 when another scale is added in the combination search, but the time required to complete a full search with the added scale is increased by a factor of 2. This result stems from the fact that the total number of combinations to be searched is 2.sup.n -1, where "n" is the number of scales.
As indicated in our co-pending application, Ser. No. 430,354, filed Sept. 30, 1982, a full search of all combinations is not essential to arrive at the combination of weigh scales which most closely approximate the target amount. For example, combinations which include subcombinations equal to or greater than the target amount will not constitute improvements over the subcombinations themselves, and therefore, certain steps in a search sequence which builds upon subcombinations can be skipped or omitted in the search operation. Since the number of steps which can be skipped in any given cycle depends upon the weights of product in the scales, the number of steps skipped varies from cycle to cycle, and the time required to complete a search operation is not predictable.
Since the time required to complete the search operation varies from one machine cycle to the next and is not predictable, combination weighing machines which are designed to always complete a combination search generally allot for every search a maximum time, that is, the time necessary for all combinations to be examined. Since most of the search operations can be completed in a substantially shorter period of time, machines which allot the maximum time waste some time in each cycle of operation while the alloted search time runs out.
In the combination weighing machine in our co-pending application Ser. No. 572,661 filed Jan. 20, 1984 and entitled "Combination Weighing Machine with Adaptive Signal Correction", only qualified scales are searched, and the number of qualified scales participating in search operation varies from one cycle to the next. Some scales are periodically withdrawn from operation for purposes of calibration. Other scales which were recently dumped may be in the process of reloading while a new search is conducted on the remaining scales. Still other scales may be inoperative. Each change of a participating scale also changes the required search time by a factor of two, as mentioned above. Thus, those machines which allot a maximum period of time to complete a full search operation waste a substantial portion of the search time.
In certain prior art combination weighing machines, the number of scales participating in the combination search was arbitrarily limited in order to reduce the search time. At the same time, search time was being wasted because the time allotted was based upon the maximum time needed to conduct a full search. It has been determined that the amount of give-away is improved (reduced) only insignificantly by conducting a full search as opposed to cutting off the search in one-sixth of the allotted time. Furthermore, the amount of "give-away" is also improved by increasing the number of scales that participate in the search operation. Therefore, the overall performance of the combination weighing machine, taking into consideration accuracy and speed, can be improved by allotting less than the maximum period of time for the combination search and including as many scales as possible in the search. A limitation on the period of time to conduct a search results in some machine cycles in which the search operation is terminated before the complete search is performed, but this consequence can be made to be insignificant and in many cycles, the full search will be carried out in any event due to the skipping of certain combinations containing subcombinations exceeding the target amount and due to the fact that not all scales are prepared or qualified to participate in a search.
It is accordingly a general object of the present invention to provide a combination weighing machine and a method of operating that machine in which the combination search is terminated whether the search is completed or not, and the results of the partial search are then utilized to form a charge of product.
It is a further object of the present invention to select from the search results after termination of preferred combination on the basis of criteria that were not employed during the search itself.